Photo-mask blank comprising a shading layer having a variable etch rate

ABSTRACT

In a photo-mask blank comprising a transparent substrate member and a shading layer of chromium covered on a principal surface of the substrate member, the shading layer has a first portion adjacent the principal surface and etched at a first etch rate and a second portion farther from the principal surface than the first portion and etched at a second etch rate lower than the first etch rate. For this purpose, nitrogen is dispersed so that the first portion becomes rich in nitrogen as compared with the second portion. Alternatively, carbon is dispersed so that the first portion becomes scarce in carbon relative to the second portion. The first portion may include nitrogen. The substrate member may comprise a transparent flat plate and a transparent conductive layer coated on the flat plate. The photo-mask blank is processed into a photo-mask through an etching process.

This is a division of application Ser. No. 552,156 filed 11/15/83, nowU.S. Pat. No. 4,563,407.

BACKGROUND OF THE INVENTION

This invention relates to a photo-mask blank for use in manufacturing asemiconductor device, an integrated circuit, a large-scale integratedcircuit, and the like and to a photo-mask made from the photo-maskblank.

A conventional photo-mask blank of the type described comprises atransparent substrate member having a principal surface and a shadinglayer of chromium (hereafter frequently referred to as a chromium layer)coated on the principal surface. The transparent substrate member maycomprise a transparent plate and a transparent conductive layer attachedto the transparent plate. The transparent plate may be a soda-limeglass, borosilicate glass, silica glass, or sapphire while thetransparent conductive layer is made of indium oxide, tin oxide, or thelike. The shading or chromium layer is frequently covered with anantireflection layer. The chromium layer and the chromium oxide layerare formed by sputtering, ion plating, or the like.

At any rate, the chromium layer is selectively etched by an etchant witha photoresist covered on the photo-mask blank, so as to leave apredetermined pattern of the chromium on the principal surface.

It should be noted here that objectionable dust is inevitably attachedto the photoresist when the photoresist is made to adhere to thephoto-mask blank by coating and baking the photoresist on the photo-maskblank. Such objectionable dust can be eliminated together with thephotoresist on removal of the photoresist. However, the dustunpleasingly gives rise to undesired spots of chromium on the substratemember after exposure of the photoresist. This is because thephotoresist is not exposed at the portions to which the dust isattached. The spots become serious when a fine pattern is delineated asthe predetermined pattern.

The spots may be removed by carrying out excessive etching oroveretching because they are very small. However, the excessive etchingadversely affects the predetermined pattern and makes it difficult todelineate a precise pattern.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a photo-mask blank which iscapable of delineating a fine pattern.

It is another object of this invention to provide a photo-mask blank ofthe type described, wherein undesired spots of chromium can be reducedwhen the photo-mask blank is etched into a photo-mask.

It is a further object of this invention to provide a photo-mask blankof the type described, wherein the undesired spots can rapidly be etchedoff.

It is a yet further object of this invention to provide a photo-maskwhich is substantially free from the undesired spots.

A photo-mask blank to which this invention is applicable is for use in alithograp technique. The photo-mask blank comprises a transparentsubstrate member having a principal surface and a shading layer ofchromium having a first surface brought into contact with the principalsurface and a second surface opposite the first surface. The shadinglayer comprises a first portion which is adjacent the first surface andwhich provides the first surface and a first etch rate and a secondportion which is farther from the first surface than the first portionand which provides the second surface and a second etch rate lower thanthe first etch rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c show a sectional view for use in describing a method ofprocessing a conventional photo-mask blank into a photo-mask;

FIG. 2 shows a sectional view for use in describing in detail an etchingprocess of the method illustrated in FIG. 1;

FIG. 3 shows a similar view for use in describing a principle of thisinvention;

FIGS. 4a, 4b show sectional view for use in describing a photo-maskblank according to each of first and second embodiments of thisinvention;

FIG. 5 shows a graph for use in describing a relationship between anitrogen atmosphere and an etch rate;

FIG. 6 shows a graph for use in describing characteristics of thephoto-mask blank according to the first embodiment of this invention andthe conventional photo-mask blank illustrated in FIG. 1;

FIG. 7 shows a graph for use in describing a relationship between acarbon atmosphere and an etch rate; and

FIG. 8 shows a graph for use in describing characteristics of thephoto-mask blank according to the second embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a method of processing a conventional photo-maskblank 10 into a photo-mask 11 will at first be described for a betterunderstanding of this invention. In FIG. 1(a), the illustratedphoto-mask blank 10 comprises a transparent substrate member 15 having aprincipal surface directed upwards, a chromium layer or a chromiumlamina 16 on the principal surface, and a chromium oxide layer 17 on thechromium layer 16. The transparent substrate member 15 may either atransparent plate or a combination of a transparent plate and atransparent conductive layer attached to the transparent plate. Thetransparent plate may be of soda-lime glass, borosilicate glass, silicaglass, or sapphire while the transparent conductive layer, indium oxide,tin oxide, or the like. The chromium lamina 16 has a first surface incontact with the principal surface and a second surface opposite thefirst surface and brought into contact with the chromium oxide layer 17.The chromium layer 16 and the chromium oxide layer 17 serve as a shadinglayer and an antireflection layer, respectively.

The chromium layer 16 is deposited to a thickness of 650 angstroms undera pressure of 10⁻³ Torr in an argon atmosphere by the use of a d.c.sputtering apparatus of a planar magnetron type. The sputteringapparatus is adjusted so that the chromium layer 16 has an opticaldensity of 3.0. Nitrogen may be dispersed in the chromium layer 16 bycarrying out sputtering in a mixed gas of argon and nitrogen. The etchrate of the chromium layer 16 is invariable.

The chromium oxide layer 17 is deposited on the chromium layer 16 to athickness of 250 angstroms under a pressure of about 1×10⁻⁴ Torr in anoxygen atmosphere by the use of the same sputtering apparatus. Theillustrated photo-mask blank has a low reflectivity because of presenceof the antireflection layer and may therefore be called a low reflectionphoto-mask blank. Nitrogen may be included in the chromium oxide layer17.

As shown in FIG. 1(a), the photo-mask blank is covered with aphotoresist 21 which may be AZ-1350 manufactured and sold by Shipley FarEast Ltd., Tokyo, Japan. The photoresist 21 is subjected to a heattreatment so as to increase adhesion of the photoresist 21 and tovaporize a solvent included in the photoresist 21.

During the heat treatment, dust can adhere to the photoresist 21, asdepicted at 22 in FIG. 1(a), because such heat treatment is generallycarried out in a comparatively dusty atmosphere of, for example, Class100. Such dust particles are 1 micron or so in size.

Under the circumstances the photoresist 21 is subjected to exposure,development, and selective removal. On selective removal of thephotoresist 21, the dust 22 itself is removed together with thephotoresist 21 but a portion of the photoresist 21 which underlies thedust 22 undesiredly remains on the photo-mask blank 10, as depicted at25 in FIG. 1(b), in addition to a desired or a predetermined photoresistpattern (not shown) because such a portion is not subjected to theexposure.

Subsequently, the photo-mask blank 10 illustrated in FIG. 1(b) isimmersed into an etchant to selectively etch the chromium layer 16 andthe chromium oxide layer 17. The etchant is formed by adding distilledwater to 165 grams of ceric ammonium nitrate and 42 milliliters ofperchloric acid (70%) and kept at a temperature between 19° C. and 20°C. The etching lasts a 50 seconds and, thereafter, the remainingphotoresist 25 is eliminated in a known manner. Thus, the photo-mask 11is manufactured with an undesired spot 26 left on the substrate member15, as shown in FIG. 1(c), together with a desired pattern (not shown).It is readily understood that the undesired spot 26 results from thedust 22 illustrated in FIG. 1(a) and comprises the chromium layer 16 andthe chromium oxide layer 17. The number of such undesired spots shouldbe reduced to a minimum number in order to delineate a fine pattern. Asknown in the art, the number of undesired spots is specified by anaverage number of undesired spots per unit area (1 cm²), which will besimply called a spot number.

The above-mentioned etching isotropically makes progress and, as aresult, the undesired spot 26 gradually becomes wide as it comes nearthe principal surface. In other words, the undesired spot 26 has amaximum width in contact with the principal surface, as illustrated inFIG. 1(c). This is true of the desired pattern as long as theisotropical etching is carried out.

Referring to FIG. 2, chromium layer 16' and a chromium oxide layer 17'are etched with a photoresist 26' attached to the chromium oxide layer17'. When etching is advanced to the principal surface the substratemember 15, both of the chromium layer 16' and the chromium oxide layer17' are etched out. An etched-out time can be determined inconsideration of the etching rate and the thickness of the chromiumlayer 16' and the chromium oxide layer 17'. When the ethcing iscontinued after the etched-out time, an undercut inevitably occurs underthe photoresist 26', as shown in FIG. 2. The etching after theetched-out time may be referred to as overetching. Such an undercut canbe specified by a width x₁ of the photoresist 26' and a maximum width x₂of the chromium layer 16' measured along the principal surface. Morespecifically, the undercut is defined by a difference between the widthx₁ of the photoresist 26' and the maximum width x₂ of the chromium layer16'.

In FIG. 1, assume the overetching is carried out with the photoresist 25coated on the chromium oxide layer 17. In this event, the undesired spot26 gradually becomes small with an increase of the undercut. Finally,the undesired spot 26 can be removed from the principal surface by theoveretching. However, such removal of the undesired spot 26 istime-consuming. For example, the time necessary for reducing the spotnumber to 0.1 (number/cm₂) is twice as long as the etched-out time. Inaddition, such long overetching makes it difficult to delineate a finepattern, as described in the Background section of the instantspecification.

Referring to FIG. 3, description will be made regarding the principle ofthis invention. In FIG. 3, a lamina 30 is coated on the principalsurface of the substrate member 15 and is etched with a photoresist 26'attached to the lamina 30 so that the lamina 30 becomes narrower inwidth at a lower principal surface near the portion than at an upperportion remote from the principal surface. In other words, the lamina 30may quickly be etched at the lower portion as compared with the upperportion. When such a lamina 30 is coated on the principal surface, anundesired spot is quickly removed from the substrate member 15 as aresult of an undercut.

In order to realize such an etch rate, consideration is directed to acomposition of the lamina 30.

Referring to FIG. 4, a photo-mask blank according to a first embodimentof this invention comprises a transparent substrate 15 similar to thatillustrated in conjunction with FIG. 1. In FIG. 4(a), the illustratedlamina 30 comprises a first chromium layer 31 brought into contact withthe principal surface of the substrate member 15 and a second chromiumlayer 32 on the first chromium layer 31. The first and the secondchromium layers 31 and 32 may be referred to as first and secondportions, respectively. A combination of the first and the secondchromium layers 31 and 32 is operable as a shading layer. The first andthe second chromium layers 31 and 32 provide first and second surfacesbrought into contact with the principal surface and the chromium oxidelayer, respectively. The first and the second chromium layers 31 and 32respectively have a first etch rate and a second etch rate lower thanthe first etch rate. To this end, the first and the second chromiumlayers 31 and 32 are deposited in a mixed gas of nitrogen and argon bythe use of a d.c. sputtering device of a planar magnetron type.

Temporarily referring to FIG. 5, curve 33 shows a relationship between amixed ratio (mole percent) of nitrogen included in the mixed gas and anetch rate (angstroms/second) of a chromium layer. Use is made of anetchant similar to that described in conjunction with FIG. 1(b). It isreadily understood from the curve 33 that the etch rate of the chromiumlayer is raised with an increase of the mixed ratio of nitrogen.Inasmuch as the content of nitrogen increases in the chromium layer withan increase of the mixed ratio of nitrogen, it may be said that the etchrate of the chromium layer is dependent on the degree of nitrogenizationof the chromium layer.

Taking the above into consideration, the first and the second chromiumlayers 31 and 32 are deposited in atmospheres different from each other.

Returning to FIG. 4, chromium oxide layer 17 is deposited as anantireflection layer on the second chromium layer 32 to a thickness of250 angstroms. The chromium oxide layer 17 may include nitrogen. Acombination of the first and the second chromium layers 31 and 32 andthe chromium oxide layer 17 is etched in a manner similar to the lamina30 illustrated in FIG. 3.

Table 1 shows Examples 1, 2, and 3 according to the first embodiment ofthis invention together with a conventional example. On manufacturingeach example, the sputtering apparatus is adjusted so that an opticaldensity becomes equal to 3.0. In each of Examples 1, 2, and 3, the firstand the second chromium layers 31 and 32 are deposited to thicknesses of150 and 500 angstroms, respectively, while the conventional example hasa chromium layer of 650 angstroms thick.

                  TABLE 1                                                         ______________________________________                                                  1st Cr layer (31)                                                                        2nd Cr layer (32)                                                  Ar:N.sub. 2                                                                              Ar:N.sub. 2                                                        (Molar ratio)                                                                            (Molar ratio)                                            ______________________________________                                        Example 1   75:25        85:15                                                Example 2   60:40        85:15                                                Example 3   40:60        85:15                                                Conventional                                                                              85:15        85:15                                                Example                                                                       ______________________________________                                    

In Examples 1 through 3, each first chromium layer 31 is etched at thefirst etch rate greater than 20 angstroms/second while etch secondchromium layer 32, the second etch rate lower than 20 angstroms/secondand higher than 18 angstroms/second, as shown in FIG. 5. In other words,the first chromium layer 31 is rich in nitrogen as compared with thesecond chromium layer 32. This means that the first chromium layer 31 ishigher in the degree of nitrogenization than the second chromium layer32. On the other hand, the chromium layer of the conventional example isetched at a uniform etch rate not higher than 18 angstroms/second.

Referring to FIG. 6, wherein the abscissa and the ordinate represent anetching time ratio of a practical etching time to an etched-out time anda spot number described in conjunction with FIG. 1(c), respectively,curves 35₁, 35₂, and 35₃ show characteristics of Examples 1, 2, and 3,respectively, while curve 36 shows the characteristic of theconventional example.

It is preferable that the spot number is not greater than 0.1/cm², asmentioned before. In order to reduce the spot number to 0.1/cm², theconventional example takes twice the etched-out time as shown by thecurve 36 whereas the embodiments of the invention take, 1.4 times aslong as the etched-out time at most, as readily understood from thecurves 35₁ to 35₃. In addition, the reduction of the spot number becomesrapid in relation to the degree of the nitrogenization included in thefirst chromium layer 31. As a result, it is possible to make the etchingtime ratio approach unity. Accordingly, the combination of the differentetch rates of the chromium layers serves as a means for rapidly reducingthe number of undesired spots of the chromium layers remaining on thesubstrate by undercutting as a result of etching.

The degree of nitrogenization may continuously be varied within thefirst and the second chromium layers 31 and 32 so that the etch rate isgradually and continuously reduced as a distance from the principalsurface increases.

Referring to FIG. 4 again and also to FIG. 7, a photo-mask blankaccording to a second embodiment of this invention comprises a firstchromium layer 31 deposited to a thickness of 150 angstroms in a firstmixed gas of argon and nitrogen and a second chromium layer 32 depositedto a thickness of 500 angstroms in a second mixed gas of argon andmethane (CH₄). The first and the second chromium layers 31 and 32therefore include nitrogen and carbon, respectively. The contents ofnitrogen and carbon included in the first and the second chromium layers31 and 32 are specified by a degree of nitrogenization and a degree ofcarbonization, respectively. Thereafter, a chromium oxide layer 17 isdeposited to a thickness of 250 angstroms on the second chromium layer32 in a known manner. The chromium oxide layer 17 serves as anantireflection layer. Nitrogen may be included in the chromium oxidelayer 17 by carrying out sputtering in an atmosphere of, for example,argon and nitrogen monoxide. A combination of the first and the secondchromium layers 31 and 32 and the chromium oxide layer can be etched ina similar manner to the lamina 30 illustrated in FIG. 3.

In FIG. 7, a first curve 41 shows a relationship between a first mixedratio (mole percent) of nitrogen included in the first mixed gas and afirst etch rate of chromium layer including the nitrogen. The firstcurve 41 is similar to the curve 33 illustrated in FIG. 5 and thereforemanifests that the etch rate is raised with an increase of the nitrogenincluded in the mixed gas. A second curve 42 shows the relationshipbetween a second mixed ratio of methane included in the second mixed gasand a second etch rate of a chromium layer including the carbon. Thesecond etch rate decreases with an increase of the second mixed ratioand is less than 5 angstroms/second when the second mixed ratio exceeds10%.

Table 2 shows Examples 4, 5, and 6 according to the second embodiment ofthis invention. In carrying Examples 4, 5, and 6, the sputteringapparatus is adjusted so that optical density becomes equal to 3.0 ineach shading layer comprising the first and the second chromium layers31 and 32.

                  TABLE 2                                                         ______________________________________                                                  1st Cr layer (31)                                                                        2nd Cr layer (32)                                                  Ar:N.sub. 2                                                                              Ar:CH.sub. 4                                                       (Molar ratio)                                                                            (Molar ratio)                                            ______________________________________                                        Example 4   80:20        88:12                                                Example 5   60:40        88:12                                                Example 6   40:60        88:12                                                ______________________________________                                    

It is readily seen from Table 2 and FIG. 7 that each first chromiumlayer 31 of Examples 4 to 6 is etched at an etch rate greater than 20angstroms/second while each second chromium layer 32 thereof is etchedat an etch rate less than 5 angstroms/second. Consequently, there isachieved a ratio of the etch rate of the first layer to that of thesecond layer of at least 4. At any rate, the shading layer is slowlyetched at a portion remote from the principal surface and is quicklyetched at another portion adjacent to the principal surface.

Referring to FIG. 8, wherein the abscissa and the ordinate represent anetching time ratio of practical etching time to etched-out time and aspot number undesirably remaining on the substrate member 15,respectively, curves 44, 45, and 46 show characteristics of Examples 4,5, and 6, respectively. It is seen from curves 44, 45, and 46 that thespot number can be reduced to 0.1/cm² before the etching times ratiobecomes 1.4 time the etched-out time. Thus, the undesired spot asillustrated in FIG. 1(c) can quickly be eliminated from the principalsurface. In addition, the practical etching time becomes short with anincrease of the nitrogen included in each first chromium layer 31 andapproaches the etched-out time.

While this invention has thus far been described in conjunction with afew embodiments thereof, it will readily be possible for those skilledin the art to put this invention into practive in various manners. Forexample, the antireflection layer 17 may not always be attached to theshading layer. Carbon along may be distributed to the chromium layer sothat the etch rate of the chromium layer becomes low as the distancefrom the principal surface increases. In this event, the second chromiumlayer 32 is rich in carbon as compared with the first chromium layer 31.In other words, the second chromium layer 32 is higher in the degree ofcarbonization than the first chromium layer 31. Instead of sputtering,vapor deposition, ion-plating, or the like may be used to deposit theshading and the antireflection layers.

What is claimed is:
 1. A photo-mask blank for use in a lithographytechnique, comprising a transparent substrate member having a principalsurface and a shading layer of chromium having a first surface incontact with said principal surface and a second surface opposite saidfirst surface, said shading layer comprising:a first portion which isadjacent said principal surface and which provides said first surface,said first portion having a first etch rate; and a second portion whichis farther from said first surface than said first portion and whichprovides said second surface, said second portion having a second etchrate lower than first etch rate to provide in combination therewith ameans for rapidly reducing undesired spots of the chromium layerremaining on the substrate to a number which is not greater than 0.1/cm²wherein the difference in etch time between the first and secondportions is sufficiently great to effect reduction in a time periodwhich is not more than 1.4 times greater than the time period forproducing etching without undercutting.
 2. A photo-mask blank as claimedin claim 1 wherein said difference in etch time between the first andsecond portions is so great that the ratio of the increase in the timeperiod of etching approaches unity.
 3. A photo-mask blank as claimed inclaim 1 wherein the ratio of etch time of the seond layer to that of thefirst layer is at least
 4. 4. A photo-mask for use in lithographytechnique, comprising a transparent substrate member having a principalsurface and a patterned shading layer of chromium having a first surfacein contact with said principal surface and a second surface oppositesaid first surface, said patterned shading layer comprising:a firstportion which is adjacent said principal surface and which provides saidfirst surface, said first portion having a first etch rate; and a secondportion which is farther from said first surface than said first portionand which provides said second surface, said second portion having asecond etch rate lower than said first etch rate to provide incombination therewith a means for rapidly reducing undesired spots ofthe chromium layer remaining on the substrate to a number which is notgreater than 0.1/cm² wherein the difference in etch time between thefirst and second portions is sufficiently great to effect reduction in atime period which is not more than 1.4 time greater than the time periodfor producing etching without undercutting.
 5. A photo-mask as claimedin claim 4 wherein said difference in etch time between the first andsecond portions is so great that the ratio of the increase in the timeperiod of etching approaches unity.
 6. A photo-mask as claimed in claim2 wherein the ratio of etch time of the second layer to that of thefirst layer is at least 4.